OLEDs are light emitting devices which use organic electroluminescent (EL) materials excited by electric current to emit light. A conventional OLED structure is described in an article by C. W. Tang and S. A. VanSlyke, "Organic Electroluminescent Diode",Applied Physics Letters, Vol. 51, 913 (1987) ("Tang & VanSlyke 1987"). Tang and VanSlyke describe a device in which an indium tin oxide (ITO) anode is placed on a glass substrate and covered successively with a hole transport layer composed of an aromatic diamine, a luminescent layer composed of tris-(8-hydroxyquinoline) aluminum (Alq.sub.3) and a metallic cathode composed of a magnesium/silver (Mg:Ag) alloy. EL light is emitted by the recombination of holes and electrons in the luminescent Alq.sub.3 layer. In this structure, the EL light is emitted down through the hole transport and ITO layers and the glass substrate.
Conventional OLEDs, however, suffer from several drawbacks and limitations.
In order to use OLEDs in a display application, it is necessary to integrate the OLEDs with circuitry, such as active matrix display drivers, which will power and control the OLEDs. Such control and power circuitry can be based on different technologies, some of which deliver positive and others negative voltage to the OLED. For technology which delivers negative voltage to the OLED (such as amorphous Si NFETs), it is necessary to have the OLED cathode in contact with the circuit's driving element. The bottom terminal of a conventional OLED, however, is an anode. This limits the ability to integrate negative voltage technology control circuitry with conventional OLEDs.
Conventional OLEDs also have long-term reliability problems. Degradation mechanisms of OLEDs have been extensively investigated recently. It is commonly understood that the exposure of a conventional OLED to air while the device is in operation accelerates device degradation, as described in P. E. Burrows et al. "Reliability and Degradation of Organic Light Emitting Devices", Applied Physics Letters, Vol. 65, 2922 (1994). More specifically, degradation mechanisms involving the top metal electrode of conventional OLEDs have been observed.
Another problem experienced with conventional OLEDs is that a significant portion of the EL light generated in the OLED is not emitted in a direction normal to the device, but is rather waveguided in the substrate, which must be substantially transparent, or in the organic films. This problem is described in D. Z. Garbuzov et al., "Photoluminescence Efficiency and Absorption of Aluminum-tris-quinolate (Alq.sub.3) Thin Films", Chemical Physics Letters, Vol. 249, 433 (1996). The waveguided light does not reach the observer standing in front of the OLED and is effectively lost, thereby reducing the effective efficiency of the OLED.